Hi, ftp uses more ports that two(20,21). It uses random ports for passive connections. You must open/forward more ports..range of ports and configure these ports on pureftpd server. Or you can turn on on your router DMZ function --> All queries will be forwarded to IP in DMZ setting. For example:

that is if your server is on LAN side. (IF you have public IP that you can optionaly use active mode). IF your server is on WAN side you must have turned on passive mode in your ftp client

BTW, this post more fall to general that ispconfig....moderator, would you move this post?